In hard disk drives, data is written to and read from magnetic recording media, herein called disks, utilizing magnetoresistive (MR) transducers commonly referred to as MR heads. Typically, one or more disks having a thin film of magnetic material coated thereon are rotatably mounted on a spindle. An MR head mounted on an actuator arm is positioned in close proximity to the disk surface to write data to and read data from the disk surface.
During operation of the disk drive, the actuator arm moves the MR head to the desired radial position on the surface of the rotating disk where the MR head electromagnetically writes data to the disk and senses magnetic field signal changes to read data from the disk. Usually, the MR head is integrally mounted in a carrier or support referred to as a slider. The slider generally serves to mechanically support the MR head and any electrical connections between the MR head and the disk drive. The slider is aerodynamically shaped, which allows it to fly over and maintain a uniform distance from the surface of the rotating disk.
Typically, an MR head includes an MR read element to read recorded data from the disk and an inductive write element to write the data to the disk. The read element includes a thin layer of magnetoresistive sensor stripe sandwiched between two magnetic shields that are electrically connected together but are otherwise isolated. A constant current is passed through the sensor stripe, and the resistance of the magnetoresistive stripe varies in response to a previously recorded magnetic pattern on the disk. In this way, a corresponding varying voltage is detected across the sensor stripe. The magnetic shields help the sensor stripe to focus on a narrow region of the magnetic medium, hence improving the spatial resolution of the read head.
Earlier MR sensors operated on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the read element resistance varied as the square of the cosine of the angle between the magnetization and the direction of sense current flowing through the read element. In this manner, because the magnetic field of the recording media would effect the magnetization direction within the read element, the change in resistance could be monitored to determine the type of external magnetic field applied by the magnetic recording medium. Most current disk drive products utilize a different, more pronounced magnetoresistive effect known as the GMR or spin valve effect. This effect utilizes a layered magnetic sensor that also has a change in resistance based on the application of an external magnetic field. While multiple layers are typically used, the most relevant layers are a pair of ferromagnetic layers separated by an electrically conductive non-magnetic spacer layer such as copper. One of the ferromagnetic layers known as the “free” layer is a soft magnetic material whose magnetization is changed by the external magnetic field caused by the close proximity of the magnetic recording medium. The other ferromagnetic layer, known as the “pinned” layer, is also a soft magnetic material that has its magnetization direction fixed by an adjacent layer known as the “pinning” layer. A layer of antiferromagnetic material is typically used as the pinning layer. A sense current is passed from one end of the ferromagnetic and conductive layers to the opposite end of those same layers. The resistance of this tri-layer structure is proportional to the cosine of the magnetization angle between the two ferromagnetic layers. Since one of the layers has a magnetization angle that is pinned and the other ferromagnetic layer has a magnetization that can vary in response to the magnetic field from an adjacent magnetic recording medium, the resistance of the tri-layer structure is a function of that magnetic field from the recording medium. It has been discovered that this tri-layer structure behaves in this manner because of a spin dependent scattering of electrons, the scattering being dependent on the spin of the electron and the magnetization direction of the layer through which the electron passes.
Competitive pressures within the computer industry require progressively increasing storage capacity within a given footprint for a disk drive. To provide this increased storage capacity, it is necessary to increase the areal density of data stored on the magnetic media. Of course, increasing areal density drives other constraints. It is necessary for the sensitivity or output of the read sensor to be increased in order to compensate for the smaller flux levels provided from the smaller area on the media where a given bit of data is recorded. Second, it is necessary for the read/write head to be able to write to and read from progressively smaller areas. For the read element, this means a narrower read gap.
For a GMR read element, the read gap is defined by the shield-to-shield spacing. Unfortunately, due to the finite thickness of the GMR film therebetween and the gap coverage on either side for electrical isolation, GMR read element designs appear to be approaching a hard limit of 45 to 50 nanometers. It is not at all clear how read gaps smaller than this can be designed for GMR read elements.
Perpendicular recording of data has been proposed to solve some of these issues. See U.S. Pat. No. RE 33,949, entitled “Vertical Magnetic Recording Arrangement”, the contents of which are incorporated herein by reference. Perpendicular recording can both increase the area of density of recorded data and increase flux levels. One of the inherent characteristics of perpendicular recording, however, is that the read head always senses a magnetic field, as compared to longitudinal recording where the magnetic field is only sensed when the direction of magnetic recording changes. For this reason, perpendicular recording techniques proposed to date have differentiated the output from the read element. To date, differentiation has been performed electronically and has typically resulted in a loss of 3-5 dB SNR.
Differential GMR read elements have also been proposed. If problems relating to differential GMR readings could be solved, they could both help to enable perpendicular recording techniques in commercial products and decrease the effective read gap width. A differential GMR read element includes two different GMR read sensors located adjacent to each other. With their free layers separated by an insulation layer, the effective read gap width of the differential GMR read element is the distance from the magnetic center of one free layer to the magnetic center of the other free layer. As can be appreciated, this distance is much smaller than the shield-to-shield spacing. Unfortunately, all differential read elements proposed to date have suffered from various problems. In order to provide a differential output, the differential read elements of one prior art device were designed to have the magnetization direction of the adjacent free layers point in opposite directions. Unfortunately, this is difficult to do in practice.
As can be seen, there are many challenges that remain to be resolved before differential GMR sensors are commercially feasible. It is against this background and a desire to improve on the prior art that the present invention has been developed.